Richt was highly successful at UGA when the SEC really was he superior conference (unlike the hype it is now). He happened to coach in that conference at a time when two of the best all timers in the game were dominating it.
Richt's biggest issue was winning the big ones. Most years he had no problem winning the other games. Kelly isn't beating the Northwestern's and Duke's of the world.
The two really aren't in the same class, and Richt wasn't good enough. If that tells you anything.